SP-3760 Guard against infinite loop page trees

[jonls]

Guard against infinite loops in the page tree.

I've not been able to find any PDFs with deeper nesting than 10 but someone in pdfrw issues mentioned that a particular tool was creating a subtree for each page so depth would be equal to the number of pages. This seems to be rare though and I've set the max depth to 50000 so I think it should be unlikely to run into this threshold.

Another solution would be to keep track of all Pages objects that have been seen already (this is what poppler does) which means we could instantly detect cycles. This would be relatively easy if we could tell the object number but I don't think it's possible with pdfrw.

Testing:

• I am still able to read all PDFs in https://github.com/pmaupin/static_pdfs
• I can read the user PDF that previously got stuck in an infinite loop. It breaks out of the loop almost instantly

JIRA Tickets
SP-3760

[hyangda]

Awesome, maybe we can look into keeping track of seen objects in the master repo for Hacktoberfest